file area listings: Since these file don't contain paths, the paths are blank
(which is normal and expected), but a recent change treated directories with
the same path (even if blank) as the same directory and over-write. Now, only
non-blank paths are compared to see if the directory is a duplicate and should
be overwritten.

Also changed references to FILEBONE.NA to FILEGATE.ZXX since that appears to
be the currently distributed filename. Update help text to reflect this too.

Also display a popup error if attempting to import a non-existant filename.